In recent years, global warming due to an increase in carbon dioxide in the air is one of global environmental issues. Regarding vehicles, it is positively coped with size and weight reductions for an improvement of fuel efficiency. In fact, the amount of use materials is reduced in accordance with an improvement of a yield rate of raw materials for products. With this, the amount of carbon dioxide generated during a manufacturing process of the products can be reduced.
Because the production amount of vehicle parts is large, the generation of the carbon dioxide is largely reduced by coping with the size and weight reduction of the parts and improving the yield rate. For example, in a vehicle starter, which is one of electromagnetic devices, a starter motor is relatively heavy. The size and weight reduction and the improvement of material yield of the starter motor contributes to the reduction of the carbon dioxide. The size and weight reduction of the starter motor has been coped so far. However, it has not been coped sufficiently with the improvement of the yield rate.
In general, a d.c. commutating motor is used for the starter motor. A yield rate of a rotor core is very low in the starter motor. Conventionally, the rotor core is formed by a stack of thin steel sheets. The sheet is produced by stamping a square-shaped steel sheet with a predetermined dimension. When the round-shaped rotor is formed of the square-shaped steel sheet, the yield rate is π/4. In fact, approximately 40% to 50% of the material is discarded.
For example, Japanese Patent Publication No. JP-B2-61-11065 proposes a manufacturing method of a rotor of a rotary electric machine for improving the yield rate of the sheet. Generally, a stator of the rotary electric machine that has a ring-shaped cross-section, especially, a stator core of a revolving-field type electric machine, is produced by helically winding a sheet in a form of belt. In JP-B2-61-11065, this method is applied to make the rotor core of a rotary electric machine. Specifically, the belt sheet for the rotor core is in a form of comb and has long teeth. The belt sheet is produced from a base metal sheet. In the base metal sheet, two belt sheets are opposed and arranged such that the teeth of a first sheet are intermeshed with the teeth of a second sheet. The rotor core is produced by helically winding one of the first and second sheets.
At the ends of the teeth, notches are formed. The belt sheets are arranged in the base metal sheet such that the teeth are opposed to each other. That is, the notches of the first sheet do not mate with the notches of the second sheet. As a result, the material around the notches is discarded. Therefore, it is difficult to fully reduce scraps of the material. Also, in a belt sheet used for making a core in which side walls of the teeth defining slots are parallel or a core having a large number of slots such as a rotor core of a starter, especially, a magnet type, it is difficult to arrange two belt sheets in the base metal sheet such that the teeth of the first sheet are intermeshed with the teeth of the second sheet.
FIG. 10 shows an example of a base metal sheet for a rotor core of a starter. In the base sheet, two sheets are arranged such that the teeth are opposed to each other. Each sheet is helically wound as shown in FIG. 11. However, the teeth of the first sheet are not intermeshed with the teeth of the second sheet. Therefore, it is difficult to improve a yield rate in this method. Accordingly, the sheets shown in FIG. 10 are not widely used. Also, in the sheet-used in JP-B2-61-11065, since the projections formed on the side opposite to the teeth are short, it is difficult to helically wind this sheet.